1. Field of the Invention
The present invention relates to oilfield threaded connections of the type having an engaging torque shoulder. More particularly, the present invention relates to an oilfield threaded connection in which the thread load flank is at a slightly negative angle, and the torque shoulder is at a similar negative angle. This combination minimizes the loss of preload when high pressure develops in the connection due to entrapped thread compound.
2. Description of the Related Art
Offshore drilling using automated handling and makeup systems have led to connections which are prepared with thread compound either on the shore base or on the rig prior to being taken up to the draw works for running the tubular string downhole. While the above techniques desirably place employees out of harms way, these techniques preclude the ability to apply the “human touch” as to proper thread compound application. Accordingly, the connection's sensitivity to having excess thread compound applied is significantly higher with the modern approaches of running pipe.
Threaded connections with seals adjacent each end for both internal and external pressure sealing tend to trap the thread compound between the seals. Seals are desirable to obtain the capabilities needed for pressure reversal scenarios. Inadvertently, however, these seals also trap the pressure of the thread compound which is trying to escape the connection during makeup. Moreover, integral expanded box and liner connections, which commonly have this configuration, are oft times less robust than threaded and coupled connections due to limited material available driven by clearance issues which mandate their use.
Integral, tapered, two-step, negative load flank, dual seal connections exist in the market. One example is the Grant Prideco ANJO connection which has a negative load flank thread form, two-step thread, a 15° torque shoulder near the axial center of the connector, and both internal and external seals. Another connection is the Hydril SLX connection which also has a negative load flank thread form, two-step threads, a 15° torque shoulder near the axial center of the connector, and internal and external seals. For this latter case, the external seal is also mid-connector. A third connection with a negative load flank thread form and two-step threads is the VAM SLIJII connection, which has a torque shoulder near the axial center of the connector, substantially perpendicular to the axis of the connection, and also has internal and external seals. Integral, tapered, single step, negative load flank, dual seal connections also exist in the market. One example is the Hunting Seal Lock SF, which has a negative load flank thread form, single step threads, a 15° torque shoulder near the external face of the connector, and internal and external seals. Other similar connectors with flush or near flush outer diameter also exist.
One of the potential significant adverse effects of excess thread compound generating high pressures within the connector is loss of connection preload and stored torque when the compound pressure dissipates. The loss of stored torque is of significant concern in offshore drilling applications where the connector has the potential for unscrewing and disengaging during the running process. Making up a connection up to 30,000 ft. lbs and having it break out at 5,000 ft. lbs is particularly disconcerting for operators drilling 100 million dollar wells in 6,000 feet of water. By maintaining substantially the makeup torque and the preload, the reliability of the connection may be significantly improved.
U.S. Pat. No. 4,753,460 discloses a threaded connection with a negative load flank on two-step threads, a positive torque shoulder on the pin connector, and the box connector for high torque engagement. U.S. Publication 2002/0033603 discloses a threaded connection with conventional threads with rounded roots and a load shoulder between the end of the box and a mating surface on the pin. An end surface of the box member may also engage a mating surface on the pin member.
The disadvantages of the prior art are overcome by the present invention, and an improved oilfield tubular connection is hereinafter disclosed.